Date: Thu, 31 Oct 2002 15:18:47 -0700 (MST) From: "M. Warner Losh" <imp@bsdimp.com> To: tlambert2@mindspring.com Cc: bde@zeta.org.au, rittle@labs.mot.com, rittle@latour.rsch.comm.mot.com, current@FreeBSD.ORG, dschultz@uclink.Berkeley.EDU Subject: Re: Lack of real long double support Message-ID: <20021031.151847.03097281.imp@bsdimp.com> In-Reply-To: <3DC17FC5.AF56552E@mindspring.com> References: <3DC0E0A7.290A57CA@mindspring.com> <20021031.013338.106483974.imp@bsdimp.com> <3DC17FC5.AF56552E@mindspring.com>
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In message: <3DC17FC5.AF56552E@mindspring.com>
Terry Lambert <tlambert2@mindspring.com> writes:
: "M. Warner Losh" wrote:
: > : I await an explanation of how you can fit 2*DBL_MAX into a double,
: > : which has a range of DBL_MIN..DBL_MAX.
: >
: > Look at the code.
: >
: > long double a = DBL_MAX;
: > long double b = DBL_MAX * 2;
: >
: > The original posting said that b would be +Inf at this point, which is
: > not correct. I think that Bruce was confused there. The more correct
: > example to look at was the one that rittle@ posted which was 1 +
: > LDBL_EPSILON.
:
: I guess I must not be understanding. What will b be, at this point,
: then? How can it have a value larger than DBL_MAX that's not +Inf?
:
: If it's possible to represent a value larger than DBL_MAX in a double,
: then the value of DBL_MAX is wrong, right? Maximum means maximum,
: doesn't it?
*LONG*DOUBLE* is not *DOUBLE*. long double has extended precision and
a range compared to double. That's how.
: > : I think that a number that's a 64 bit mantissa reaised to an exponent
: > : N takes a larger N if you have only 53 bits of mantissa, if you want
: > : to represent the same value.
: >
: > Nope. The only difference between 53 bits and 64 bits of precision is
: > just that: precision. The number of bits for expoentent is
: > independent of this.
:
: .125 ^ 2 = 0.015625
: .25 ^ 3 = 0.015625
:
: So if I go from 3 digits of precision to 2 digits of precision for
: my mantissa, in order to represent the same number, I need a larger
: exponent.
That's not how it works. The exponent is more like
.1250000 * 2^3
vs
.1249999 * 2^3
Both have exponent 3, but the differ by a bit or two in the mantissa.
Warner
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